1. Field of the Invention
The present invention relates to a capacitor device, and more particularly, to a supercapacitor formed of new material having the characteristics of high capacitance and high power.
2. Description of the Related Art
Batteries are usually used as energy storage devices for systems such as portable electronic equipment and electric automobiles requiring an independent power supply unit or systems which adjust instantaneously occurring overload or supply energy. However, approaches of using capacitors instead of batteries have been attempted to improve the input and output characteristics of stored electric energy in terms of electric power, that is, in terms of input and output of energy.
Capacitors have better input and output characteristics of stored electric energy than batteries and are semipermanent such that they may be reused a considerably greater number of times, i.e., more than one hundred thousand times, than the average number of times, i.e., 500 times, which batteries are usually used. Representative conventional capacitors, i.e., condensers, have capacitance on the order of only xcexcF or pF, so they are restrictively limited. Due to new materials developed since the early 1990s, supercapacitors such as electrochemical capacitors having capacitance of more than several tens of F and holding the merits of existing capacitors have been developed. Supercapacitors cover electrochemical capacitors, electric double layer capacitors and ultracapacitors.
Such a supercapacitor uses an electrode of activated carbon or activated carbon fiber having a specific surface of about 1000-2000 m2/g to increase energy storage volume, that is, capacitance. It is known that a capacitor using an electrode of activated carbon or activated carbon fiber has capacitance per surface area of about 10-15 xcexcF/cm2.
Supercapacitors have an energy density of about 1-10 Wh/kg, which is one tenth of secondary cells"" energy density of about 20-100 Wh/kg. Here, the energy density indicates the energy storage volume per weight. However, supercapacitors have a power density of 1000-2000 W/kg, which is ten times higher than secondary cells"" power density of 50-200 W/kg. Here, the power density indicates the volume of accumulated electric energy which can be supplied per unit time. Therefore, supercapacitors are expected to function as electric energy storage devices or load controllers in place of secondary cells. To meet this expectation, it is required to increase the capacitance of supercapacitors to the level of secondary cells.
Although activated carbon or activated carbon fiber has a relatively large specific surface as described above, its pores have a diameter of 20 xc3x85 or less so that ions cannot easily enter the pores. Accordingly, supercapacitors using an electrode formed of activated carbon or activated carbon fiber have a limitation in increasing capacitance.
To solve the above problems, it is an object of the present invention to provide a supercapacitor using an electrode of a new material, the supercapacitor having higher capacitance.
Accordingly, to achieve the above object, there is provided a supercapacitor including two electrodes facing each other, the electrodes being composed of carbon nanotubes, an electrolyte provided between the two electrodes, and a separator for separating the electrolyte between the two electrodes.
For each of the electrodes, the carbon nanotubes mixed with a bonding agent are molded into a pallet pattern. Here, the carbon nanotubes may be single-wall or multi-wall carbon nanotubes. Alternatively, each of the electrodes is formed of carbon nanotubes which are vertically grown on respective collectors.
Meanwhile, the carbon nanotubes may be activated by a solution containing potassium hydroxide. Alternatively, the carbon nanotubes may be electrolessly plated with nickel, or Raney nickel may be applied to the carbon nanotubes.
According to the present invention, a high performance supercapacitor having high capacitance and low internal resistance can be provided.